The invention relates to a light transmitter comprising at least two semiconductor crystals, in each of which an electroluminescent junction is formed between two regions of opposite conductivity types, the said crystals being accommodated in a transparent protective envelope which has a surface from which the radiation emitted by the said junctions emanates. The junctions can be alternately energized from an external current source through two output electrodes which are passed through the said transparent envelope.
Electroluminescent diodes are frequently used for visual display, in particular in data processing technology. A logic state in binary form can be visualized by means of a diode which assumes two states: illuminated diode or, extinguished diode. However, in the case of a strong environmental illumination, the reflection effects on the optical properties of the said diodes may give rise to interpretation errors. For this reason, different logic states are preferably visualized by means of diodes which emit in different wave lengths, for example, in the red and in the green range.
Two-colored light elements are already known which consist of two semiconductor crystals, one of which emits in the red range at a given polarity, while the other at the opposite polarity emits in the green range, as shown in U.S. Pat. No. 3,873,979.
The crystals used are of the p/n type, i.e. crystals in which in a body of the n-conductivity type a doped zone of the p-conductivity type is present, the light emanating from the latter zone. However, use of these crystals in light transmitters leads to technological difficulties. For reasons of rationalization, it is to be preferred that such devices comprise only two electrodes. In order that the region of n-conductivity type of one of the crystals will be in electrical contact with the region of p-conductivity type of the other crystal, it is necessary that the n-type regions are insulated from each other, that is to say that their metal supports are also insulated from each other. This condition consequently leads to the use of two separate supports.
Such a construction is complicated and not very economical and yields a moderate result. The crystals are in fact necessarily arranged eccentrically with respect to the longitudinal axis of the light element and with the use of two separate supports it is difficult to keep this eccentricity low.
The said crystals are enveloped by a colorless resin, but in order to reduce the eccentricity effect of the crystals, this resin must contain a diffuse constituent which gives a less agreeable gray shade and which in the presence of a strong environmental illumination does not improve the contrast.
The electroluminescent diodes may also be used outside the visible spectrum for more specific applications, in particular in the field of the information transmission. In the case in which they are at least joined together pairwise in the same device, difficulties are met which are substantially similar to those mentioned above.
The known devices have other disadvantages due to the fact that they must comprise only two output electrodes, whereas originally each of the crystals also comprise two electrodes which are each connected to a semiconductor region. The first of the electrodes of each crystal consists of the metal support of the said crystal and the second electrode is a simple connection tongue. During construction of the device, the first electrode of one of the crystals must be electrically connected to the second electrode of the other crystal and conversely, the distance between the crystal supports and the crystals themselves should be kept constant.
The construction of such light transmitters consequently involves a number of difficulties, only a moderate product being obtained at a comparatively high cost, with and a mass production being difficult.